APPLE
01234
ZERO-INDEX
LENGTH: 5
String str = "APPLE";
charAt(n) method
number(zero-index)
char ch0 = str.charAt(0); ==> 'A'
char ch1 = str.charAt(1); ==> 'P'
char ch2 = str.charAt(4); ==> 'E'
char ch3 = str.charAt(5); ==> StringIndexOutOfException
